From this discussion [1], it seems that the two are not supported simultaneously to avoid confusion. The two options don't actually conflict in any way, but some users get confused by assuming that everything is thread-safe when the flag is enabled, rather than just the C API.

I have a pretty serious need for the C++ interface to HDF5, so I was hoping you can include it despite this minor issue. You could always add a warning when the package is installed saying that only C API is thread-safe, not FORTRAN, nor C++.